Fire Simulation completely disappears from scene (2.8)
You need to bake the simulation in Smoke > Cache otherwise it won't be saved to disk. If you don't, the simulation will be recalculated the next time you open the file and play the animation. This can be time consuming, since it would need to be done every time you open the file again. If you already have a cached animation from hitting play, you can also press Current Cache to Bake which will stored the cached animation, but not calculate it for the following frames that haven't been simulated yet.